Safety control device, safety control system, and method for detecting communication error

ABSTRACT

A safety control device for ensuring safety of an object to be controlled, including: communication unit which is used for communication with another safety control device and uses a frame including a test bit string T having correlation with a base bit string B; and detection unit which compares with the base bit string B the test bit string T included in a frame having been received from another safety control device, thereby determining the number of bit errors in the frame.

CROSS REFERENCE TO RELATED APPLICATIONS

This application contains subject matter related to Japanese andInternational applications JP2004-221736, JP2004-222101,PCT/JP2005/013888 and PCT/JP2005/013889, the entire contents of whichbeing incorporated herein by reference in their entirety. Priorityapplications PCT/JP2005/013888 and JP2004-222101 were filed on Jul. 22,2005 and Jul. 29, 2004 respectively.

TECHNICAL FIELD

The present invention relates to a technique for detecting acommunication error between safety control devices which ensure safetyof an object to be controlled.

BACKGROUND ART

Conventionally, control apparatuses, such as programmable controllers(PLCs), have been employed in factory automation (FA) for controllingmechanical devices. In recent years, such a control apparatus is desiredto have a safety function of stopping a mechanical device when ananomaly occurs, to thus ensure safety. Hence, safety control devicesequipped with such a safety function have been proposed (see, e.g.,JP-A-2002-358106).

In FA, a plurality of control apparatuses are connected by way ofcommunication lines and form a network system, to thus performsimultaneous control of a plurality of mechanical devices. Hence, acommunication error which arises between safety control devices must bedetected accurately for enhancing the reliability of the safetyfunction.

Meanwhile, among safety control devices forming a network system asdescribed above, communication is carried out with use of frames inaccordance with, e.g., the high-level data link control (HDLC) protocol.During the course of communication, a communication error is detected bya parity check or by a cyclic redundancy check (CRC), and corrected.

DISCLOSURE OF THE INVENTION

The related-art parity check or the CRC can detect presence/absence of abit error in a frame/however, the number of bits with errors cannot bedetected. Accordingly, the number of bit errors which have occurred in apredetermined period of time cannot be determined accurately. Thiscauses a bottleneck in enhancing the reliability of the safety function.

The present invention has been conceived in view of the above problem,and aims at providing a safety control device whose safety function ishighly reliable, as well as a safety control system.

The invention also aims at providing a method for detecting acommunication error which can enhance reliability of a safety functionprovided by a safety control device.

The invention defined in claim 1 is a safety control device for ensuringsafety of an object to be controlled, including: a communication unitwhich is used for communication with another safety control device anduses a frame including a test bit string having correlation with a basebit string; and a detection unit which compares the test bit stringincluded in the frame having been received from another safety controldevice with the base bit string, thereby determining the number of biterrors in the frame. In the invention, the test bit string hascorrelation with the base bit string. Accordingly, when the test bitstring included in a frame having been received from another safetycontrol device is compared with the base bit string in consideration ofthe correlation, the number of bit errors in the received frame can bedetermined accurately. Hence, since communication errors are detectedaccurately, reliability of a safety function for ensuring safety of anobject to be controlled can be enhanced.

According to the invention defined in claims 2 and 4, at least a portionof the test bit string is a copied bit string of the base bit string.Accordingly, the number of bit errors can be determined easily by meansof merely performing bit comparison between the base bit string and thecopied bit string of the test bit string included in the received frame.Hence, a time required for comparing the test bit string with the basebit string for determining the number of bit errors is reduced.

When an error due to a stack out, in which a certain bit is fixed to “0”or “1,” has arisen in a copied bit string of a test bit string includedin a received frame, detection of the bit error itself through bitcomparison between the copied bit string and a base bit string sometimesfails.

According to the invention defined in claims 3 and 4, at least a portionof the test bit string is a reversed bit string of the base bit string.Therefore, even when an error due to a stack out has occurred in the bitstring of the reversed bit string of the test portion included in thereceived frame, the error can be detected without fail by means of bitcomparison between a re-reversed bit string—which is a re-inversion ofthe reversed bit string—with the base bit string. Therefore, the numberof bit errors can be accurately determined.

Even when a bit error has occurred in a test bit string in a receivedframe, there may be a case where no change arises in the correlationbetween the test bit string and a base bit string. In this case,detecting the bit error through comparison between the test bit stringand the base bit string becomes impossible.

According to the invention defined in claim 5, the base bit string andthe test string vary with time while maintaining mutual correlation.Accordingly, even when detection of a bit error has failed at a certainpoint in time, the bit error can be detected by means of comparisonbetween a base bit string and a test bit string, each of which hasvaried with time after the failure. Therefore, accuracy in determinationof the number of bit errors can be enhanced.

According to the invention defined in claim 6, the frame includes thebase bit string and the test bit string. Accordingly, the base bitstring can be used in common among safety control devices which carryout communication, irrespective of time-varying changes in the base bitstring. In addition, in such a case where an additional safety controldevice is to be connected to a system which is configured such that aplurality of safety control devices are connected together, a base bitstring which is common to all the safety control devices can beembodied.

The invention defined in claim 7 further includes counting unit forincrementing a count value every configured cycle. The base bit stringindicates the count value of the counting unit. Accordingly, the basebit string varies every configured cycle, whereby the test bit stringhaving correlation with the base bit string also varies every configuredcycle. Hence, comparison between the base bit string and the test bitstring, which vary with time while maintaining mutual correlation, canbe attained by means of a comparatively simple method.

The invention defined in claim 8 further includes storage unit forstoring an accumulated value of the number of bit errors detected by thedetection unit. Accordingly, the number of bit errors having occurredwithin, e.g., a predetermined period of time, can be determinedaccurately.

The inventions defined in claims 9 and 10 further includesafety-ensuring unit which performs, when the accumulated value of thenumber of bit errors within a set period of time exceeds an allowablevalue, control for ensuring safety of the object to be controlled.Accordingly, when bit errors occur in a number exceeding the allowablevalue, safety of the object to be controlled can be ensuredinstantaneously, whereby the reliability of the safety function can beensured. The invention defined in claim 11 is a safety control systemconfigured such that a plurality of safety control devices defined inany one of claims 1 to 10 are connected by way of communication lines.Accordingly, effects of the safety control device defined in any one ofclaims 1 to 10 constituting the safety control system can be yielded.

The invention defined in claim 12 is a method by means of which a safetycontrol device for ensuring safety of an object to be controlled detectsan error in communication with another safety control device, including:a receiving step of receiving, from another safety control device, aframe including a test bit string having correlation with a base bitstring; and a detection step of comparing with the base bit string thetest bit string included in the frame having been received in thereceiving step, thereby determining the number of bit errors in theframe. In the invention, the test bit string has correlation with thebase bit string. Accordingly, when the test bit string included in theframe having been received from another safety control device iscompared with the base bit string in consideration of the correlation,the number of bit errors in the received frame can be determinedaccurately. Hence, since a communication error is detected accurately,the reliability of a safety function for ensuring safety of an object tobe controlled can be enhanced.

The invention defined in claim 13 is a method for detecting acommunication error among a plurality of safety control devices forensuring safety of an object to be controlled, including: a generationstep of, in a first safety control device, generating a frame so thatthe frame includes a test bit string having correlation with a base bitstring; a transmission step of transmitting to a second safety controldevice the frame generated by the first safety control device; and adetection step of, in the second safety control device, comparing withthe base bit string the test bit string included in the frame havingbeen received from the first safety control device, thereby determiningthe number of bit errors in the frame. In the invention, the test bitstring has correlation with the base bit string. Accordingly, when, inthe second safety control device, the test bit string included in theframe having been received from the first safety control device iscompared with the base bit string in consideration of the correlation,the number of bit errors in the received frame can be determinedaccurately. Hence, since a communication error is detected accurately,the safety function for ensuring safety of an object to be controlledcan be provided more reliably.

Meanwhile, in the inventions defined in claims 1 to 13, the term “bitstring” is to be understood as encompassing data of one bit in additionto data whose bit length is two or larger.

In addition, the invention defined in claim 13 may be practiced suchthat at least one of a plurality of safety control devices is selectedas the first safety control device, and at least another one of the sameis selected as the second safety control device. In this case, theapparatuses selected as the first safety control device and as thesecond safety control device may be sequentially replaced with lapse oftime.

BRIEF DESCRIPTION OF THE DRAWING

In the accompanying drawings:

FIG. 1 is a schematic view for explaining a frame for use in anembodiment of the present invention;

FIG. 2 is a block diagram showing a safety control system according tothe embodiment of the invention/

FIG. 3 is a block diagram showing a safety control device according tothe embodiment of the invention; and

FIG. 4 is a flowchart for describing processing of a communication erroraccording to the embodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinbelow, an embodiment of the present invention will be described byreference to the drawings.

FIG. 2 shows a safety control system 1 according to an embodiment of theinvention. The safety control system 1 is a network system configuredsuch that a plurality of safety control devices are connected in a ringconnection by way of communication lines 2, such as optical fibercables. Meanwhile, the maximum number of safety control devicesconnected in the safety control system 1 of the embodiment is set to 24.However, for convenience of explanation, the following description isbased on an assumption that three safety control devices are connectedtherein.

In the safety control system 1, the safety control device denoted withreference numeral 10 functions as a master device, and the remainingsafety control devices denoted with reference numerals 11 and 12function as slave devices. An input device 3, such as an emergencybutton or a safety sensor, and an output device 4, such as a motor or arobot, are connected to each of the safety control devices 10 to 12 byway of communication lines 5. By means of cooperative operation of therespective safety control devices 10 to 12, the safety control system 1controls the respective output devices 4 on the basis of input datasupplied from the respective input devices 3, thereby ensuring safety ofthe respective output devices 4.

As shown in FIGS. 2 and 3, each of the safety control devices 10 to 12includes a communication module 20, an input/output module 30, and a busmodule 40.

As shown in FIG. 3, the communication module 20 includes an MPU 22, RAM23, and an HDLC controller 24, which are connected with each other byway of an internal bus 21; and an interface 25 connected to the HDLCcontroller 24.

The MPU 22 is connected to the input/output module 30 by way of the busmodule 40. The MPU 22, which has ROM 27, executes a program stored inthe ROM 27, thereby controlling the HDLC controller 24 and theinput/output module 30. In particular, the ROM 27 in the safety controldevice 10 stores a sequence program written in a ladder language. TheMPU 22 of the safety control device 10 executes the sequence program,thereby controlling the entire safety control system 1. Morespecifically, the safety control device 10 can be considered aprogrammable controller (PLC); and the safety control system 1 includingthe same can be said to be a PLC system. In the RAM 23, a variety ofdata—such as a count value and the number of bit errors—exchangedbetween the MPU 22 and the HDLC controller 24, and between the MPU 22and the input/output module 30 are stored sequentially.

The interface 25 has an output connector 28 and an input connector 29.The output connector 28 is connected, by way of a communication line 2,to one of the other two safety control devices other than that on whichthe output connector 28 is disposed. The input connector 29 is connectedto the other one of the safety control devices by way of anothercommunication line 2.

The HDLC controller 24 generates a frame in conformance with the HDLCprotocol, and transmits the thus-generated frame to a safety controldevice-connected to the output connector 28. The HDLC controller 24analyze and processes, among frames received from a safety controldevice connected with the input connector 29, a frame which includes aportion indicating the safety control device on which the HDLCcontroller 24 is disposed as the destination of the frame. By means ofutilizing a result of this analysis, the HDLC controller 24 processes acommunication error. In addition, the HDLC controller 24 transmits tothe safety control device connected with the output connector 28, amongthe received frames, a frame which includes, as the destination of theframe, a safety control device other than the safety control deviceequipped with the HDLC controller 24.

The input/output module 30 is connected to the input device 3 and theoutput device 4 by way of the communication lines 5. Upon receipt of acommand from the MPU 22 connected by way of the bus module 40, theinput/output module 30 supplies to the MPU 22 data pertaining to theinput device 3. Upon receipt of a command from the MPU 22, theinput/output module 30 powers-on/powers-off the output device 4.

In the safety control system 1 configured as above, the MPU 22 of thesafety control device 10 manages communication of the entire safetycontrol system 1 in accordance with the sequence program of the ROM 27.Under this management, the respective safety control devices 10 to 12carry out communication with use of frames, in a predetermined order percycle.

Heretofore, an overview of the safety control system 1 has beendescribed.

Hereinbelow, features of the safety control system 1 will be describedin more detail.

First, a frame for use in communication among the respective safetycontrol devices 10 to 12 will be described in detail.

As schematically shown in FIG. 1, the frame includes two flag sequencesF; a destination address portion DA; a source address portion SA; aframe-type portion FT; a bit length portion L; an information portion I;a test portion T; and a frame check sequence FCS.

The flag sequence F is a bit string of one byte, and defined as“01111110” in the HDLC protocol. The destination address portion DA is abit string of one byte indicating an address of a safety control deviceserving as the destination of the frame. The source address portion SAis a bit string of one byte indicating an address of a safety controldevice serving as the source of the frame. The frame-type portion FT isa bit string of two bytes indicating a frame type defined by acombination of: a command description for the destination of the frame,a serial number of the frame, information pertaining to occurrence of aserial failure, and the like. The bit length portion L is a bit stringof two bytes indicating a total bit length of the information portion Iand the test portion T.

The information portion I is a bit string of one byte or larger whichindicates a plurality of control data sets to be supplied to thedestination of the frame. In the present embodiment, the last one byteof the information portion I is a base bit string B which indicates,among control data sets to be supplied to the destination of the frame,a count value stored in the RAM 23 of the source of the frame. The testportion T is a test bit string of two bytes, and includes a copied bitstring C, which is a copy of the base bit string B/and a reversed bitstring R, which is an inversion of the base bit string B. The framecheck sequence FCS is a bit string of two bytes indicating data for theCRC.

Next, generation of a frame by the HDLC controller 24 of one of thesafety control devices 10 to 12 will be described.

Upon receipt of a command from the MPU 22, the HDLC controller 24 in thesafety control device 10, 11, or 12 executes frame-generationprocessing. More specifically, the HDLC controller 24 generates thedestination address portion DA, the source address portion SA, and theframe-type portion FT so as to respectively indicate a destinationaddress, a source address, and a frame type supplied from the MPU 22.

In addition, the HDLC controller 24 generates the base bit string B sothat the base bit string B indicates a count value of the RAM 23 at astart time of processing. The thus-generated base bit string B is mergedwith a bit string indicating control data supplied from the MPU 22,thereby generating the information portion I. Simultaneously, the HDLCcontroller 24 generates the copied bit string C, which is a copy of thebase bit string B, and the reversed bit string R, which is an inversionof the base bit string B, thereby generating the test portion Tconstituted of the bit strings C and R. Furthermore, the HDLC controller24 generates the bit length portion L from the thus-generatedinformation portion I and the test portion T.

Still furthermore, the HDLC 24 generates the frame check sequence FCSfrom the thus-generated respective portions DA, SA, FT, I, T, and L.

Next, analysis of a frame by the HDLC controller 24 of each of thesafety control devices 10 to 12 will be described.

Upon receipt of a frame to be analyzed, the HDLC controller 24 in thecorresponding one of the safety control device 10 to 12 executesframe-analysis processing. More specifically, the HDLC controller 24extracts a frame type from the frame-type portion FT of the frame to beanalyzed, and supplies the thus-extracted frame type to the MPU 22. Inaddition, the HDLC controller 24 extracts data for the CRC from theframe check sequence FCS of the frame to be analyzed, and executes theCRC by utilization of the thus-extracted data.

Furthermore, the HDLC controller 24 extracts the copied bit string C andthe reversed bit string R from the test portion T of the frame to beanalyzed, and supplies the MPU 22 with the thus-extracted bit strings Cand R. Simultaneously, the HDLC controller 24 extracts the base bitstring B from the information portion I of the frame to be analyzed, andsupplies the MPU 22 with the thus-extracted base bit string B.

Next, a method for processing a communication error executed by thesafety control devices 10 to 12 will be described by reference to aflowchart shown in FIG. 4. In FIG. 4, the safety control device 10 isdenoted as a master device, and the safety control devices 11 and 12 aredenoted as slave devices.

In each of the safety control device 10 to 12, when the bit strings C,R, and B are supplied from the HDLC controller 24, the MPU 22 executesprocessing of a communication error. More specifically, the MPU 22performs bit comparison between the copied bit string C and the base bitstring B, sequentially from the highest order bit or from the lowestorder bit. Simultaneously, the MPU 22 performs bit comparison between are-reversed bit string R′, which is a re-inversion of the reversed bitstring R, and the base bit string B, sequentially from the highest orderbit or from the lowest order bit (step S1). When, consequently, a bitwhich fails to match at least one of the bit strings C and R′ isdetected in the reference bit B (step S2), the MPU 22 stores thecumulative number of the thus-detected bits in the RAM 23 as the numberof bit errors (step S3). This storage of the number of bit errors intothe RAM 23 is performed in such a manner as to add the number of errorbits onto the number of bit errors having been stored in advance.Meanwhile, the MPU 22 also has a function of updating the number of biterrors stored in the RAM 23 every set period of time. Accordingly, thenumber of bit errors stored in the RAM 23 is an accumulated value withinthe set period of time. As described above, the MPU 22 monitors thenumber of bit errors stored in the RAM 23. When the accumulated value ofthe number of bit errors in the set time; that is, a bit error rate,exceeds an allowable value (step S4), the MPU 22 determines that aserious failure has occurred (step S5 and S9).

When the MPU 22 of the safety control device 10 determines that aserious failure has occurred (step S5), the MPU 22 powers off the outputdevice 4 connected to the input/output module 30 of the same safetycontrol device 10 (step S[beta]). In conjunction therewith, the MPU 22of the safety control device 10 causes the HDLC controller 24 togenerate and transmit frames including the frame-type portion FT whichindicates a frame type commanding power-off of the output devices 4, andthe destination address portion DA whose destination addresses are thesafety control devices 11 and 12 (step S6). Consequently, in each of thesafety control devices 11 and 12 which has received the frame, thecommand to power-off the output device 4 is extracted from theframe-type portion FT and supplied to the MPU 22 in accordance with ananalysis performed by the HDLC controller 24 (step S7). The MPU 22powers-off the output device 4 connected to the input/output module 30(step S8).

When the MPU 22 determines that a serious failure has occurred in thesafety control device 11 or 12 (step S9), the MPU 22 causes the HDLCcontroller 24 to generate and transmit a frame including the frame-typeportion FT which indicates a frame type indicating occurrence of theserious failure, and the destination address portion DA whosedestination address is the safety control device 10 (step S10).Consequently, in the safety control device 10 which has received theframe, data pertaining to occurrence of the serious failure is extractedfrom the frame-type portion FT and supplied to the MPU 22 in accordancewith analysis performed by the HDLC controller 24 (step SI1). The sameprocessing as in the case where the MPU 22 has determined that a seriousfailure has occurred is executed (steps S6 to 8). Therefore, all theoutput devices 4 connected to the input/output modules 30 of therespective safety control devices 10 to 12 are powered-off.

As described above, the HDLC controllers 24 of the respective safetycontrol devices 10 to 12 correspond to communication unit; the MPUs 22of the same correspond to detection unit and safety-ensuring unit; andpieces of RAM 23 and the MPUs 22 correspond to storage unit.

Next, storage of a count value in the RAM 23 of each the safety controldevices 10 to 12 will be described.

In the safety control device 10, the MPU 22 increments a count value ofthe RAM 23 every time the MPU 22 starts one communication cycle.

In each of the safety control devices 11 and 12, the HDLC controller 24performs analysis of the information portion I of the frame to beanalyzed having been received from the safety control device 10 asdescribed above, thereby extracting the base bit string B. The MPU 22stores in the RAM 23 a count value indicated by the thus-extracted basebit string B. Accordingly, count values to be stored in the pieces ofRAM 23 of the safety control devices 11 and 12 are basically thosehaving been incremented every configured cycle.

Thus, the MPUs 22 of the respective safety control devices 10 to 12correspond to counting unit. The process of steps S1-5 and S9corresponds to a method for detecting an error according to the presentinvention.

The above-described safety control system 1 uses a frame including thetest portion T formed from the copied bit string C of the base bitstring B, and the reversed bit string R of the base bit string B duringthe course of communication among the respective safety control devices10 to 12. Accordingly, each of the safety control devices 10 to 12performs bit comparison between the copied bit string C of the testportion T of a received frame and the base bit string B, therebyattaining immediate determination of the number of bit errors havingoccurred in the frame. In addition, even when a bit error due to a stackout has occurred in the bit string C of the test portion T of thereceived frame, to thus hinder detection of the number of errors bymeans of bit comparison between the copied bit string C and the base bitstring B, determination of the number of bit errors can be attained bymeans of performing bit comparison between the re-reversed bit string R′of the reversed bit string R and the base bit string B. As describedabove, each of the safety control devices 10 to 12 compares therespective bit strings C and R of the test portion T included in areceived frame in accordance with correlation with respect to the basebit string B, thereby attaining accurate detection of the number of biterrors included in the frame.

Furthermore, each of the safety control devices 10 to 12 of the safetycontrol system 1 generates a base bit string B of the frame so that thereference bit B indicates a count value to be incremented everyconfigured cycle. Therefore, the base bit string B varies with time; andaccordingly, the test portion T generated from the copied bit string Cof the base bit string B, and the reversed bit string R of the base bitstring B also vary with time while maintaining correlation with the basebit string B. Hence, in such a case where, in spite of occurrence of abit error in the bit strings C and R of the test portion T in thereceived frame, at a certain point in time, correlation between the bitstring C and the base bit string B, or that between the bit string R andthe base bit string B, does not exhibit any change, whereby errordetection is prevented, the error can be detected by means of comparinga base bit string B which has subsequently varied with time, andrespective bit strings C and R which have also varied with time. Inother words, accuracy in determination of the number of bit errors isenhanced.

Furthermore, according to the safety control system 1, when any one ofthe safety control devices 10 to 12 determines occurrence of such aserious failure that the accumulated value of the number of bit errorsin a set period of time exceeds an allowable value, all the outputdevices 4 connected to the respective safety control devices 10 to 12are powered-off. Accordingly, safety of all the output devices 4 to becontrolled is secured instantaneously.

As described above, according to the safety control system 1, accuracyin determination of the number of bit errors which representscommunication errors is improved; and, furthermore, safety of all theoutput devices 4 can be secured instantaneously by utilization of theresult of the detection. Hence, high reliability of safety function canbe ensured.

Moreover, a frame including the base bit string B, in addition to thetest portion T, is utilized in communication among the respective safetycontrol devices 10 to 12 of the safety control system 1. Accordingly,the base bit string B which varies with time as described above can beused in common by all the safety control devices 10 to 12. In addition,in such a case where another safety control device of similarconfiguration with the safety control device 11 or 12 is to beadditionally connected to the safety control system 1 shown in FIG. 2, abase bit string B which is common to all the safety control devices canbe embodied without interrupting processing before connection.

Hithertofore, an embodiment of the present invention has been described;however, the present invention should not be understood to be limited tothe embodiment. For instance, in the above-described embodiment, thesingle input device 3 and the single output device 4 are connected toeach of the safety control devices 10 to 12 respectively. However, thenumber of input devices and output devices to be connected to the safetycontrol device can be set arbitrarily.

In addition, in the embodiment, of the safety control devices 10 to 12,only the safety control device 10 functioning as a master device isconfigured as a PLC. However, the safety control device functioning as aslave device may also be configured as a PLC.

Furthermore, in the above-mentioned embodiment, the communication module20 and the input/output module 30 of each of the safety control devices10 to 12 are configured as separate modules connected by way of the busmodule 40. However, such a communication module and an input/outputmodule may be configured as a single module.

Still furthermore, in the embodiment, the test portion T serving as atest bit string is formed from a copied bit string C of the base bitstring B, and the reversed bit string R of the base bit string B. Forinstance, however, the test portion T may be formed from only one of thecopied bit string C and the reversed bit string R. Alternatively, thetest portion T may be configured such that the test portion T is formedfrom only the copied bit string C at a certain point in time, and thesame is formed from only the reversed bit string R at another point intime. Even when a bit string which constitutes the test portion T isformed so as to vary with lapse of time, the test portion T can beembodied by means of a comparatively easy method.

INDUSTRIAL APPLICABILITY

As described above, according to the invention, there is provided asafety control device whose safety function is highly reliable, as wellas a safety control system. The invention also provides a method fordetecting a communication error which can enhance reliability of asafety function provided by a safety control device.

1. A safety control device for ensuring safety of an object to becontrolled, comprising: a communication unit that communicates a frameincluding a test bit string having correlation with a base bit stringwith another safety control device; and a detection unit that comparesthe test bit string included in the frame received from the anothersafety control device with the base bit string, and detects a number ofbit errors in the frame.
 2. The safety control device according to claim1, wherein at least a portion of the test bit string is a copied bitstring copied from the base bit string.
 3. The safety control deviceaccording to either one of claims 1 and 2, wherein at least a portion ofthe test bit string is a reversed bit string reversed from the base bitstring.
 4. The safety control device according to claim 1, wherein thetest bit string includes: a copied bit string copied from the base bitstring; and a reversed bit string reversed from the base bit string. 5.The safety control device according to any one of claims 1-4, whereinthe base bit string and the test string vary with time while maintainingmutual correlation.
 6. The safety control device according to any one ofclaims 1-5, wherein the frame includes the base bit string and the testbit string.
 7. The safety control device according to any one of claims1-6 further comprising a counting unit that increments a count valueevery configured cycle, wherein the base bit string indicates the countvalue.
 8. The safety control device according to any one of claims 1-7further comprising a storage unit that stores an accumulated value ofthe number of bit errors determined by the detection unit.
 9. The safetycontrol device according to claim 8 further comprising a safety-ensuringunit that performs, when the accumulated value within a set period oftime exceeds an allowable value, a control for ensuring safety of theobject to be controlled.
 10. The safety control device according toclaim 9, wherein the storage unit updates the accumulated value everythe set period of time.
 11. A safety control system comprising: aplurality of safety control devices for ensuring safety of objects to becontrolled; and a communication line that connects the safety controldevices with each other, wherein each of the safety control devicesincludes: a communication unit that communicates a frame including atest bit string having correlation with a base bit string with at leastanother one of the safety control devices through the communicationline; and a detection unit that compares the test bit string included inthe frame received from the another safety control device with the basebit string, and determines a number of bit errors in the frame.
 12. Amethod for detecting an error in communication between safety controldevices that ensures safety of an object to be controlled, the methodcomprising: receiving by one of the safety control devices from anotherof the safety control devices a frame including a test bit string havingcorrelation with a base bit string; and detecting a number of bit errorsin the frame by comparing the test bit string included in the framereceived from the another safety control device with the base bitstring.
 13. A method for detecting an error in communication betweensafety control devices that ensures safety of an object to becontrolled, the method comprising: generating, in a first safety controldevice, a frame including a test bit string having correlation with abase bit string; transmitting the frame generated from the first safetycontrol device to a second safety control device; and detecting, in thesecond safety control device, a number of bit errors in the frame bycomparing the base bit string with the test bit string included in theframe.